Systems have been developed to obtain an auditory evoked response (AER) or brainstem auditory evoked response (BAER) for a patient representing activity of the patient's auditory system. The AER is an electrical brain wave or neural response obtained from electrodes placed on the patient in response to a stimulus, normally a sound. Depending of the latency of the response and the placement of the electrodes, different classes or types of AERs can be obtained. Those with the shortest latency are generated by the inner ear and the auditory nerve, and are referred to as electrocochleography responses. The next response reflects activity within the auditory brainstem and is referred to as an auditory brainstem response (ABR). Further detail is provided in Hall, James W, III; Handbook of Auditory Evoked Responses; Allyn and Bacon; Needham Heights, Massachusetts, 1992.
Electrocochleography (“ECOG” or “ECochG”) systems are currently used to perform diagnoses of the cochlea and vestibular apparatus. In the case of the vestibular system, recently analysis for this specific part of the ear has been referred to as electrovestibulography (EVestG), being a specific sub-class of ECOG. The systems are used to produce a patient neural response which involves placing a recording electrode as close as practical to a patient's cochlea. An acoustic transducer, eg an earphone, is used to provide an auditory stimulus to evoke the response. For EVestG the patient is however tilted, in different directions, to evoke a specific response from the vestibular apparatus. It is not necessary to also use an auditory stimulus for EVestG. An ECOG signal representing the neural response is used to determine an Sp/Ap ratio that can be used for the diagnosis of a number of conditions, particularly Meniere's disease. The first wave, normally labelled N1, of the response signal is examined to determine the summating potential (Sp), the action potential (Ap) and the second summating potential (Sp2), as shown in FIG. 1. The response is only of the order of a few μV and is received with considerable unwanted noise making it difficult to determine and isolate.
For example, the ECOG signal is normally assessed by obtaining multiple samples from a patient in response to acoustic stimuli, and then obtaining an average Sp/Ap ratio for diagnosis. This process, however, is neither very sensitive nor specific, as a patient can have Meniere's disease and a normal ECOG, and alternatively the patient could also have an abnormal ECOG, but not have Meniere's disease. Accordingly, an alternative process (“the Franz process”) has been developed by Professor Burkhard Franz, as described in International Patent Publication WO 02/47547, which seeks to analyse directly the vestibular response, rather than the cochlea response, as Meniere's disease is a pathology of the vestibular system. The Franz process uses an ECOG system to record the response obtained from a patient asked to tilt their head either forward, backward, contralaterally or ipsilaterally. The process seeks to identify a periodic signal in the response which is believed to come from either the semi-circular canals (SCCs) or the otolith organs at predominantly 23 Hz, but also at 11.5 Hz and 46 Hz. This analysis is done by averaging the ECOG response over a number of intervals at the frequency of interest, eg 1/23 Hz at repeated intervals.
There are, however, a number of difficulties with the Franz process. Firstly, the process is not considered to be reliable for all patients, and particularly for inhibitory head tilts and especially for involuntary head tilts. The process also cannot be easily adopted by an audiologist without significant training. Also, more significantly, it has been found that the frequencies of interest, 11.5, 23 and 46 Hz, do not have characteristic signals that can be reliably located once the background signal for ambient noise has been removed. This indicates that these frequency components of the ECOG response are primarily induced by background noise and/or muscle (premotor and/or motor) activity, and any response from the SCCs and otolith organs is extremely difficult to detect or isolate at these frequencies. Similar problems exist with determining and analysing other AERs, such as the ABR.
Accordingly, it is desired to address the above, or provide at least a useful alternative.